The present invention relates to a radiography apparatus for forming a radiogram of a living body.
There has been known, as an apparatus for forming a radiogram in use for a diagnosis of a living body, an X-ray photographing apparatus, which projects X-rays into a living body from its frontal side and exposes an X-ray film disposed on the rear side of the living body. In the X-ray photograph, its spatial resolution can be improved up to desired value by merely increasing graininess of the film, but there is a limit in improving contrast resolution representing a dynamic range of a gradation because of the developing characteristics (.gamma. characteristic) of the film. Therefore, in cases requiring high contrast resolution, such as a diagnosis of lungs, heart or the like, it is almost impossible to make a precise diagnosis when the X-ray photographing apparatus is used.
For this reason, an electronic radiography apparatus has recently been developed in which an X-ray detector is disposed in place of the X-ray film and a radiogram is formed by using an output signal from the X-ray detector. Since this apparatus has components common to those of a computed tomography (CT) scanner, it is frequently assembled into the CT scanner. In the radiography apparatus, the radiogram is formed through electrical processing and is displayed on a monitor such as a CRT, that is, no photographing process is contained in forming the radiogram. Therefore, the contrast resolution of the electronic radiography apparatus is at least 15 to 20 times that of the X-ray photography apparatus. In the apparatus, however, the spatial resolution depends on the number of samples detected, so that a remarkable improvement of it is impossible. More specifically, spatial resolution corresponding to the lateral direction of a human body is determined by the number of cells in the detector which is about 1000 at most. Some portions of the objects under diagnosis partially appear on a picture frequently. In this case, when the radiogram taken is enlarged, the spatial resolution is equivalently improved. In a CT apparatus capable of taking the tomogram, a picture can be enlarged only by shortening the distance between the X-ray source and the living body. In the radiography apparatus in question, the mere shortening of the distance expands only the lateral direction of the human body. Therefore, the lateral and longitudinal expansion rates of the picture are different from each other, with the result that the obtained picture is distorted and can not be used for the diagnosis.